Conventionally, images have been recorded onto or erased from a thermally reversible recording medium according to a contact recording method of bringing a heating source into contact with the thermally reversible recording medium and heating the thermally reversible recording medium. As the heating source, a thermal head or the like is typically used for image recording, and a heating roller, a ceramic heater, or the like is typically used for image erasing. Advantageously, such a contact recording method enables uniform recording or erasing of images onto or from a thermally reversible recording medium by pressing the thermally reversible recording medium onto the heating source uniformly with a platen or the like when the thermally reversible recording medium is a flexible medium such as film and paper, and enables manufacture of an image recording apparatus and an image erasing apparatus at low costs by allowing diversion of components of printers for conventional heat-sensitive sheets.
There is a request for image rewriting to be performed to a thermally reversible recording medium from a remote position. For example, there is proposed a method of using a laser, as a method for recording and erasing images onto or from a thermally reversible recording medium uniformly, when there are undulations on the surface of the thermally reversible recording medium or from a remote place (see PTL 1). The proposed method is described to perform contactless recording to a thermally reversible recording medium pasted on a shipping container used on a distribution line, and to perform writing by a laser and erasing by hot air, hot water, or an infrared heater.
As such a recording method by a laser, there is provided a laser recording apparatus (laser marker) that irradiates a thermally reversible recording medium with high-power laser light and can control the position of the light. With this laser marker, a thermally reversible recording medium is irradiated with laser light, a photothermic material in the thermally reversible recording medium absorbs the light and converts it to heat, and recording and erasing are performed with this heat. As a method for recording and erasing images by a laser, there has been proposed a method of combining a leuco dye, a reversible developer, and various photothermic materials, and recording images by near infrared laser light (see PTL 2).
Further, use of the conventional techniques described in PTL 3 and PTL 4 enables uniform heating of a recording medium, and enables improvement of image quality and repetition durability. However, there is a problem that a time required for image recording and image erasing is long due to jumps between respective lines to be drawn, and wait times.
There is also proposed a method of detecting a surface state of a thermally reversible recording medium and controlling the irradiation energy during image recording according to the detection (see PTL 5). This proposed method enables recording of a high-quality image by controlling the irradiation energy even with respect to minute undulations, but necessitates highly precise control to bring about a problem that the cost of the apparatus will be expensive.
There is also proposed a method of adjusting an irradiation spot diameter to be constant by detecting the position of a thermally reversible recording medium and controlling the position of the lens according to the position detection result (see PTL 6). However, this proposal has a problem that the lens system for controlling the irradiation spot diameter will be complicated to raise the cost of the apparatus.
Recently, low-costing and space-saving image processing apparatuses have also been requested, and there has been proposed an image processing method of performing both of image erasing and image recording with one image processing apparatus (one laser emitting unit). In this case, the throughput is usually determined by the sum of a time taken for image erasing, a time taken for image recording, and a time taken from the end of image erasing until the start of image recording. As one method for realizing a high throughput, there is a method of reducing a time taken from the end of image erasing until the start of image recording. However, it takes time to shift from image erasing to image recording, and it has been unable to perform image rewriting at high speed. A time during which a shipping container on which a thermally reversible recording medium is pasted is conveyed, and a wait time during which the shipping container having been conveyed ceases to vibrate are not necessary, and it is only necessary to secure a time during which image erasing and image recording are switched within the image processing apparatus. Therefore, it is possible to greatly reduce the time from the end of image erasing until the start of image recording.
There is proposed an overwrite rewriting method of performing rewriting with one image processing apparatus (one laser emitting unit) (see PTL 7). This proposal describes a rewriting method of changing the beam diameter per dot between printing and erasing. However, with this proposal, it is difficult to switch the beam diameter at high speed per dot, and partial erasing may leave unerased portions if it is by rewriting per dot. Therefore, there are problems regarding image rewriting at high speed, and securement of image erasing performance.
Further, as a method for performing rewriting with one image processing apparatus (one laser emitting unit), there is proposed a method of moving the image processing apparatus or a thermally reversible recording medium to change the relative distance between the image processing apparatus and the thermally reversible recording medium (see PTL 8). However, with this proposal, it takes time to move the image processing apparatus or the thermally reversible recording medium, and it is difficult to perform rewriting at high speed.
There is also proposed a laser marking apparatus mounted with a focal length adjusting unit (see PTL 9 and PTL 10). With the focal length adjusting unit, it is possible to shift from image erasing to image recording within a short time of 1 second or shorter. At this time, heat applied to the thermally reversible recording medium for erasing the image accumulates, and this heat dissipates at short time scales. When laser light irradiation is employed as a method for applying heat, the time at which heat is applied varies from region to region within the thermally reversible recording medium, and the temperature of the thermally reversible recording medium therefore becomes non-uniform. If an image is recorded onto the thermally reversible recording medium on which the temperature is non-uniform, quenching of the thermally reversible recording layer is inhibited to thereby cause problems such as degradation of the density of an image to be drawn and degradation of repetition durability, and a region having a high temperature will be under excessive heat during image recording when an image is recorded onto the thermally reversible recording medium on which the temperature is non-uniform with a fixed laser output, to thereby thicken the line width, collapse characters and symbols, degrade the image density, and reduce readability of an information code and repetition durability.
There haven not yet been any reports on problems due to employment of an image processing apparatus mounted with the focal length adjusting unit to high speed rewriting of a thermally reversible recording medium. Such problems are more remarkable when recording plural-line drawn images to be formed by a plurality of adjacent laser light drawn lines than when recording single-line drawn images to be formed by a single adjacent-line-less drawn line. Urgent resolution of such problems is requested.
Currently available image rewriting systems in which an image erasing apparatus and an image recording apparatus are arranged side by side can perform an image erasing step and an image recording step in parallel with the image erasing apparatus and the image recording apparatus arranged side by side and are advantageous for high speed rewriting, whereas an image forming apparatus of the present invention performs an erasing step and a recording step by turns by itself, and is problematic for high speed rewriting because it necessitates a time to switch from the erasing step to the recording step. In order to realize similar processing performance to that of the currently available image rewriting systems, the image forming apparatus of the present invention needs three techniques, namely, speeding up of the erasing step, speeding up of the recording step, and reduction of the time taken to switch from the erasing step to the recording step.
Recent development of higher-power laser light sources has enabled raising of the irradiation power of laser light. By raising the irradiation power of the laser light, it has become possible to raise the temperature of the recording layer of the thermally reversible recording medium within a short time by applying energy, and to thereby realize a high speed erasing step and a high speed recording step.
However, in terms of speeding up the erasing step, not only a time during which to reach the aimed temperature but also a heating time for which the aimed temperature is maintained are necessary for erasing, and it is impossible to realize high speed erasing only by raising the irradiation power. When a spot diameter is d and a scanning velocity is V, heating time is expressed as d/V. Therefore, as a method for speeding up the erasing step, it is possible to increase a heating time for which a position is kept heated, by increasing the spot diameter of the laser light during the erasing step. Therefore, it is necessary to realize high speed erasing, by increasing the spot diameter d to thereby maintain the heating time constant even when the scanning velocity V is increased as is necessitated for speeding up.
As for image recording, in order to realize precise image formation during image recording and to secure room of margin for fluctuation of a work distance, it is preferable to control a focal length to be achieved at the position of the thermally reversible recording medium, with the focal length adjusting unit. However, there are problems regarding high speed recording, and degradation of repetition durability due to damages to the thermally reversible recording medium depending on the position thereof at which it is to be irradiated with laser light having high energy density because the beam diameter becomes small when the focal length is at the position. Meanwhile, reduction of the time taken to switch from the erasing step to the recording step is also a necessary technique.
Hence, in order to realize a space-saving image processing apparatus, it is necessary to perform high speed rewriting with one image processing apparatus (one laser emitting unit), and to perform image recording immediately after image erasing. However, a sufficiently satisfactory apparatus has not been provided yet.